Introduction to Water
Water, the most common substance on Earth's surface. It also covers more than 70 percent of Earth's surface area and exists as vapor in Earth's atmosphere. It is essential to life and greatly influences weather and climate. Water plays an essential role in many chemical reactions.
Water is used to provide hydroelectric and steam power. In rivers, lakes, and oceans it forms important avenues of transportation. As rain and ice, it wears down hills and mountains, and helps convert rocks into soil. We drink water, and use it in cooking, washing, heating, fighting fires, treating diseases, and many other ways.
The first living things appeared in water. For a very long time there was no life on earth except in water; life is still more abundant in water than on land. All living things are composed chiefly of water. An adult man, for example, consists of 65 to 75 per cent water. Water carries food to body cells and carries wastes away from them. It also helps the body maintain an even temperature. The loss of more than 15 to 20 per cent of the body's water usually results in death.
Water has symbolic meanings in many religions. In Christian baptism, water is used to symbolize the washing away of sin. To Jews, water is a symbol of purity; to Hindus, a symbol of fertility. Taoists regard water as the symbol of humility, because it always seeks the lowest place.
Interesting facts about waterHow much water is on the earth? There are about 326 million cubic miles (1.4 billion cubic kilometers) of water. There are over a million million (1,000,000,000,000) gallons of water per cubic mile (0.9 million million liters per cubic kilometer).How much of the earth's water is fresh? Only about 3 percent of the earth's water is fresh. About three-fourths of the earth's fresh water is frozen in icecaps and other glaciers. Glaciers contain as much water as flows in all the earth's rivers in about 1,000 years.How much water do living things contain? All living things consist mostly of water. For example, the body of a human being is usually 50 to 75 percent water. An elephant is about 70 percent water. A potato is about 80 percent water. A tomato is about 95 percent water.How much water does a person take in over a lifetime? On the average, a person takes in about 16,000 gallons (60,600 liters) of water during his or her life.What are the different forms of water? Water is the only substance on earth that is naturally present in three different forms--as a liquid, a solid (ice), and a gas (water vapor).How much water does a person use every day? On the average, each person in the United States uses more than 100 gallons (380 liters) of water a day in the home.What is the largest single use of water? The largest single use of water is by industry. It takes about 80 gallons (300 liters) of water to make the paper for one Sunday newspaper, and about 20 gallons of water per pound (170 liters per kilogram) of steel produced.Can water ever be used up? Water is used and reused over and over again--it is never used up. Every glass of water you drink contains molecules of water that have been used countless times before.Composition of Water
Ordinary WaterA molecule of water is composed of two atoms of hydrogen (H) and one atom of oxygen (O). Therefore the formula for water is H2O. Water is produced when hydrogen burns in the presence of oxygen. It is formed when plant and animal matter decay and when food is combined with oxygen in living organisms.
Water decomposes, or breaks down, into hydrogen and oxygen in certain high-temperature chemical reactions involving metals, or when it is electrolyzed by passage of an electric current through it.
Heavy WaterSome water molecules contain forms of hydrogen that are heavier than ordinary hydrogen. These heavier forms of hydrogen—deuterium and tritium—form molecules of water called deuterium oxide and tritium oxide, respectively.
Physical Properties of Water
Pure water is a tasteless, odorless liquid. It boils at 100 C. (212 F.), and freezes at 0 C. (32 F.). Water, like other liquids, cannot be compressed. This property makes it useful in hydraulic presses and other hydraulic devices.
The density of water is greatest at a temperature of 4 C. (39.2 F.). At this temperature, one gallon of water weighs about 8.3 pounds, one liter of water, 1 kilogram; one cubic foot of water, about 62.4 pounds; and one cubic centimeter of water, 1 gram. The maximum density of water is used as a standard of comparison for expressing the density of other liquids and solids.
Unlike most other liquids, water expands when it freezes. When water turns to ice its volume increases by a factor of 112. Since a given weight of ice occupies a greater space than an equal weight of water, ice floats in water. This physical property of water has important consequences. If water contracted on changing into a solid, ice would be heavier than an equal volume of water and would sink. The bottoms of lakes and oceans would then fill with ice, out of reach of the sun's warmth. Gradually the earth would become colder, more and more ice would form, and in time there would be little, if any, life on earth.
Specific HeatThe specific heat of water in the metric system is 1 calorie—the amount of heat required to raise the temperature of one gram one degree Celsius. (In the traditional English system it is 1 British Thermal Unit—the amount of heat required to raise the temperature of one pound one degree Fahrenheit.) Water has a higher specific heat than almost any other substance. For example, it takes almost 33 times as much heat to raise the temperature of one gram of water 1 C. as it takes to raise the temperature of one gram of gold by an equal amount. As it becomes warmer, water stores 33 times as much heat as gold; when it cools, water gives off 33 times as much heat as gold.
The high specific heat of water protects living things, which consist chiefly of water, from drastic temperature changes. It also has an important influence on weather and climate. Oceans and large lakes are cooler than nearby land in summer, and help to cool the land. In winter they slowly give off the heat absorbed in summer and help to warm the land. Currents of warm and cool ocean water greatly modify temperatures in some parts of the earth.
EvaporationLike other substances, water takes heat from its surroundings when it evaporates (changes to a vapor). A great amount of heat is required to evaporate water—about 540 calories are needed to evaporate one gram at the boiling point (100 C.). At lower temperatures even more calories are needed. At normal body temperature (98.6 F. [37 C.]), for example, about 577 calories are required to evaporate one gram of water. Thus every gram of perspiration that evaporates takes away 577 calories of heat, cooling the body. Similarly, evaporating rain and snow take vast amounts of heat away from their surroundings, and thus cool the air.
Because water absorbs so much heat in changing to a vapor, water-vapor molecules have extremely high energy. It is this property that makes steam a practical source of power.
Water vapor greatly influences climate. It absorbs part of the sun's heat by day and hinders the escape of heat from the earth by night. In deserts, where water vapor is scarce, there are extreme variations of temperature between day and night.
CondensationThe condensation (changing to liquid) of water vapor greatly affects weather and climate. In condensing, water vapor gives off the same amount of heat that was required to produce it. Heat is given off when fog, dew, and frost form, and when steam condenses in a radiator.
Freezing and MeltingWhen water at 0 C. freezes, each gram of water gives off 80 calories. The temperature of the water stays at 0 C. until every drop has frozen.
When ice at 0 C. melts, it absorbs an equal amount of heat—80 calories for each gram of ice—and its temperature remains unchanged until it has completely melted. It is by absorbing heat that ice keeps an iced drink cool.
The huge quantities of heat that are given off or absorbed when a large lake freezes or thaws have a noticeable influence on the climate of the land on its shores.
Surface Tension,caused by surface molecules clinging together and being pulled inward by other molecules, forms an apparent film on water. A needle will float when carefully placed on the surface. This tension enables water spiders and certain insects to walk on water. In using water for washing, it is necessary to reduce the surface tension to permit the water to fully wet the particles of dirt and the surface being washed; this is done with soaps and other detergents.
The Great Solvent
More things will dissolve in water than in any other known substance. For this reason pure water is never found in nature. Water in lakes, rivers, and wells always contains minerals dissolved from the earth. Even rainwater, the purest natural water, contains chemicals dissolved from the air. Pure water is obtained only by special methods of distillation and when water is produced directly from hydrogen and oxygen.
Minerals dissolved in water help nourish living things. Harmful substances, such as decaying animal and vegetable matter and poisonous chemicals, may also be dissolved, and for this reason it is important that sources of drinking water be tested. It is because water is such a good solvent, and therefore dissolves dirt, that it is used for washing.
Water that contains calcium and magnesium is said to be hard. (Soft water lacks these minerals.) Hard water, which occurs in many parts of the United States, wastes soap and forms a scale inside pipes, water heaters, kettles, and other containers. Hard water can be softened by chemical means.
Every river that runs to the sea carries minerals in solution. Some minerals, such as salt, bromine, and magnesium, can be recovered from the sea. Other minerals help nourish fish and other living things in the sea. A number of minerals and some rocks, including slate and sandstone, are obtained from the beds of prehistoric seas.
The Water Cycle
The heat of the sun vaporizes water from land and sea. Water vapor is given off also in the transpiration of plants and in the breathing of human beings and animals. Under certain conditions the water vapor in the atmosphere condenses and forms clouds. In clouds rain, snow, hail, or sleet sometimes form and, by the force of gravity, fall to earth.
Part of the water that falls on land flows along or near the surface of the ground to rivers, which carry it to the sea. This water, called run-off, is the chief cause of floods and erosion. Another part of the water is absorbed by the soil. Part of this water is evaporated by the sun, part is used by plants, and part seeps down to become groundwater.